1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system or an electrostatic recording system employed for, for instance, a printer or a copying machine, and more particularly to density control therefor.
2. Related Background Art
As an example of conventional image forming apparatuses, a color image forming apparatus of an electrophotographic system is illustrated in FIG. 8.
The color image forming apparatus has a photosensitive drum 1 as an image bearing member. The photosensitive drum 1 is rotated in the direction shown by an arrow mark by a driving means not shown. The surface of the photosensitive drum 1 is uniformly charged by a primary charging roller 2 serving as a charging means abutting against the photosensitive drum 1 during its rotation. Then, the surface of the photosensitive drum 1 is irradiated with a laser beam L in accordance with a yellow image pattern by an exposure device 3 (laser scanner) so that an electrostatic latent image is formed on the surface of the photosensitive drum 1. In this case, the charging roller 2 and the exposure device 3 serve as an electrostatic image forming means for the photosensitive drum 1.
A latent image formed on the photosensitive drum 1 is reversely developed by a developing device y with yellow toner charged with negativity contained therein, which is previously opposed to the photosensitive drum 1, as the photosensitive drum 1 rotates. On a rotary support member 5 (rotary drum) are supported four developing devices 4y, 4m, 4c and 4k. Before a development operation, a prescribed developing device is rotated and moved to a developing position opposed to the photosensitive drum 1. The latent image is visualized as a yellow toner image in accordance with the development.
The toner image obtained on the photosensitive drum 1 is transferred (primary transfer) onto the surface of an intermediate transfer belt 6 rotating in the direction shown by the arrow mark at the substantially same speed as that of the photosensitive drum 1 by a primary transfer roller 7a to which primary transfer bias is applied. The toner remaining on the surface of the photosensitive drum 1 after transfer is removed by a cleaning means such as a blade.
A process comprising charging, an exposure, a development and a primary transfer as described above is carried out for each of colors including magenta, cyan and black subsequently to yellow, hence a multicolor image obtained by superposing together the toner images of four colors is formed on the intermediate transfer belt 6.
The multicolor image formed on the intermediate color transfer belt 6 is transferred onto the surface of a transfer material serving as a recording material which is completely conveyed to the intermediate transfer belt 6 by conveying means such as pick-up rollers 9 at a prescribed timing by a secondary transfer roller 7b to which a secondary transfer bias is applied (secondary transfer).
The transfer material to which the multicolor image is conveyed to a fixing device 11 by a conveyor belt 10 in which toner is melted and fixed to the transfer material under heating and pressure, hence the multicolor changes to a final color image.
Upon the use of the image forming apparatus described above, is required such maintenance as the replenishment of toner, the treatment of waste toner, the replacement of a worn (consumed) photosensitive drum 1 by a new drum. In this example of the prior art, the photosensitive drum 1, the charging roller 2 and the cleaning means 8 are formed as an integrated process cartridge 13. Further, the developing devices 4y, 4m, 4c and 4k are each formed also as a developing process cartridge and are respectively readily detachably attachable to an apparatus main body, so that a user can perform a maintenance of them with ease.
Image forming apparatuses as well as the image forming apparatus of this example are generally provided with adjusting mechanisms for adjusting the density of an output image. Most of them have density control means for automatically controlling the output image to have proper density. Especially, in the image forming apparatus for outputting a full color image as in the present example, a more accurate density control has been demanded for each of the colors of yellow, magenta, cyan and black in order to obtain a desired color balance.
According to this example, the density of the output image is detected in such a manner that the toner image of a specific halftone pattern due to area gradation is formed on the photosensitive drum 1 and the amount of reflection light of the halftone pattern on the photosensitive drum 1 is measured by a reflection light amount sensor 12 which comprises a light emitting element and a light receiving element. Since the density of an image is controlled on the basis of image forming conditions such as the charging potential of the photosensitive drum 1, exposure potential after the exposure of laser, developing bias potential, etc., a plurality of halftone patterns are formed by changing stepwise or gradually one or the combinations of a plurality of conditions of the image forming conditions and the reflection light amount of them is respectively measured by the reflection light amount sensor 12. Thus, based on the measured reflection light amount, an image forming condition from which it is estimated that a desired constant density (reflection light amount) can be obtained is obtained.
In this connection, the reflection light amount sensor 12 employs infrared light and is designed to estimate the quantity of toner on the photosensitive drum 1 regardless of the color of toner. Although the amount of infrared light 3 received by the reflection light amount sensor 12 is substantially directly proportional to or inversely proportional to the quantity of toner sticking to the photosensitive drum 1, the quantity of toner sticking to the drum is not ordinarily proportional to the density of an output image. However, since the quantity of toner sticking to the photosensitive drum is correlated with the density of the output image in the ratio 1:1, the density of the output image can be estimated from the measured value of the reflection light amount sensor 12.
A density control for the image forming apparatus of the present example will be described in detail hereinafter. In the present example of the prior art, it is assumed that the surface of the photosensitive drum 1 is charged with electricity so that the surface potential of the photosensitive drum 1 reaches -600V and that the sensitivity of the photosensitive drum 1 and the exposure amount of laser are adjusted so that the potential of a laser exposure part reaches about -200V under normal temperature and normal humidity (23.degree. C., 60%Rh). Further, as a detecting pattern image, is used a halftone pattern (9/16) for printing 9 dots of the matrix of 4.times.4 dots as shown in FIG. 5. At this time, the developing bias formed by superimposing the AC (alternative current) voltage of rectangular wave (frequency of 2000 Hz, amplitude of 1600 Vpp) upon DC (direct current) voltage as shown in FIG. 4 is employed and a DC voltage component Vdc is changed so that the development amount of toner is controlled.
Prior to a normal image forming, as shown in FIG. 6 a plurality of image patches with the above described halftone pattern patches of square with side of 30 mm are printed at intervals in a section in which the reflection light amount sensor 12 is disposed. Each of the image patches is developed with the developing bias of a respectively different DC voltage component and the reflection light amount of each of the image patches is measured by the reflection light amount sensor 12. In this example, the number of the image patches is five and the DC voltage component Vdc of the developing bias is changed at intervals of 50V from -300V to -500V.
An example of measured results of reflection density is illustrated in FIG. 9. In this example, the target value (proper density) of the reflection density of the above described halftone pattern is set to 1.0 and an image after that is controlled to be formed based on a developing condition (in this example, the DC voltage component of the developing bias) under which the reflection density is estimated to be nearest to the target value. Consequently, the reflection density data of five points are obtained as illustrated by round marks in FIG. 9. The developing condition under which the reflection density reaches 1.0 is located in a section in which the DC voltage component Vdc exists between -400V and -450V. Assuming that a proportional relation is approximately achieved between the DC voltage component and the reflection density in this section, it may be estimated that the reflection density obtained at the time of the DC voltage component of about -420V reaches 1.0 as a result of internally dividing the reflection density at the time of the DC voltage component of -400V and that at the time of the DC voltage component of -450V. Therefore, as the image forming condition in the present example, the DC voltage component Vdc of the developing bias is controlled to -420V.
Although the number of image patches is five in the above described example, it should be noted that the number of the image patches may be increased to make the grade in change of the developing bias more minutely so that the DC voltage component of the developing bias can be accurately controlled.
The printing ratio of the halftone pattern may be changed to a different ratio so as to obtain a different density target value. However, if the printing ratio is too high or too low, the linearity between the developing bias and the density which are density variable parameters will be deteriorated, and a control value will be seldom changed, or conversely, it will be greatly changed resulting in the lack of stability. Therefore, the printing ratio of the halftone pattern which is ordinarily selected is set to 50% to 80%.
While the image forming conditions greatly depend not only on the variation in the sensitivity of the photosensitive drum 1 (variation due to temperature or humidity or durability variation), but also the unevenness in the sensitivity upon manufacturing of the photosensitive drum 1 or toner or in the charging characteristic and unevenness in the exposure amount of laser or the like, these variations can be absorbed to a certain degree and a stable image forming operation can be carried out by controlling the density as described above.
When any of the above described variation factors is large and cannot be met only by the developing bias potential, the above variation factor can be also controlled by combining the developing bias potential condition with a charging condition or an exposure condition (exposure amount).
The density control system described in the above mentioned conventional example is relatively effective for forming an image such as a photographic image including a halftone part as a main body. However, in case of an image strong in an image contrast which includes characters or graphs (an image is similar to a binary image which has few halftone parts), the above density control system has not necessarily established a proper image forming condition in view of the impression of the image. In practice, most of the images printed by a user have been images mainly including characters as in the latter case, and therefore, they have frequently encountered various problems.
After the density control described in the conventional example is carried out by employing the photosensitive drums 1 different in sensitivity, the area gradation patterns of 1/16 to 16/16 shown in FIG. 5 are printed, and the densities thereof are plotted and the plotted results are shown in FIG. 10. Referring to FIG. 10, a solid line indicates the sensitivity upon use of the photosensitive drum 1 with normal sensitivity and a broken line indicates the sensitivity upon use of the photosensitive drum 1 with high sensitivity. In this case, the surface potential of the photosensitive drum 1 is set to -600V and the exposure amount of the laser is equal to that of the conventional example.
Assuming that the surface potential of the photosensitive drum 1 in a laser exposed part is V1, V1 of the photosensitive drum 1 with normal sensitivity was approximately -200V and V1 of the photosensitive drum 1 with high sensitivity was approximately -120V. When the density control mentioned in the conventional example was applied to them, the DC voltage component Vdc of the developing bias potential selected by the photosensitive drum 1 with normal sensitivity was about -420V and the DC voltage component Vdc of the developing bias potential selected by the photosensitive drum 1 with high sensitivity was about -320V. When the difference between Vdc and V1 is represented as a developing contrast Vc for each of the photosensitive drums 1, Vc for the photosensitive drum 1 with normal sensitivity is about -220V and Vc for the photosensitive drum 1 with high sensitivity is about -200V.
As is apparent from FIG. 10, while the density on the photosensitive drum 1 with normal sensitivity substantially corresponds to that of the photosensitive drum 1 with high sensitivity in the pattern of the printing ratio of 9/16 which serves as a reference for density control, the density of the photosensitive drum 1 with high sensitivity is liable to be higher than that of the photosensitive drum 1 with normal sensitivity in the patterns having lower printing ratio and the density of the photosensitive drum 1 with normal sensitivity tends to be higher than that of the photosensitive drum 1 with high sensitivity in the patterns having the printing ratio exceeding 9/16.
The above mentioned phenomenon can be explained in the following. Since the latent image of an isolated dot on the photosensitive drum 1 with high sensitivity is deeper than that on the photosensitive drum 1 with normal sensitivity, the density on the photosensitive drum 1 with high sensitivity in the patterns low in printing ratio becomes deeper under the same developing contrast. However, as the printing ratio becomes higher, the difference in depth of the latent image between the photosensitive drum 1 with high sensitivity and the photosensitive drum 1 with normal sensitivity substantially disappears, so that the densities on the photosensitive drums 1 with high and normal sensitivity converge to the substantially same density.
The density of the photosensitive drum 1 with high sensitivity in the pattern of the printing ratio of 9/16 is slightly higher than that of the photosensitive drum 1 with normal sensitivity under the same developing contrast. However, since the density control is performed so that the density of the photosensitive drum 1 with high sensitivity corresponds to that of the photosensitive drum 1 with normal sensitivity, the developing bias potential with a slightly low developing contrast is selected. Therefore, in an image including characters or graphs having a high printing ratio, the developing contrast may possibly become insufficient, hence the characters or lines may be liable to be thinned. When the sensitivity of the photosensitive drum 1 is lowered, an action reverse to that mentioned above inconveniently operates and the developing contrast becomes more than enough so that the characters or lines tend to be thickened. Although the degree of the above tendency may be small or large, this tendency is necessarily generated regardless of the kind of toner.
Generally speaking, the sensitivity of the photosensitive drum 1 tends to be high under a high temperature and high humidity environment. On the contrary, the sensitivity of the photosensitive drum 1 tends to be low under a low temperature and low humidity environment. As the shift of sensitivity of the photosensitive drum 1 is increased, the above mentioned bad effect is apt to be more apparently generated, which has caused a problem from the viewpoint of density control.
Further, when the shift of sensitivity of the photosensitive drum is large as described above, a color balance may collapse due to the influence of the developing characteristic or the like peculiar to each color, and therefore, a method for correcting the collapse of color balance has been also demanded.
As mentioned above, not only the developing bias potential is employed as density control parameters, but also the charging potential or the exposure amount are individually adjusted, so that the quality of printing may be maintained. However, in this instance, not only a control system becomes complicated, but also density control patterns need to be repeatedly printed many times by changing settings, so that time required for control or the amount of consumed toner is increased. Therefore, a more simple and effective density control system has been required.